1. Field of the Invention
This invention relates to a process for the production of a compacted graphite cast iron containing iron, carbon and aluminum and the compacted graphite cast iron produced thereby.
2. Description of the Prior Art
Cast irons are alloys of iron and carbon, with or without other elements, usually containing 2.0 to 4.5% carbon. Where no special alloying materials are added, the final cast material is known as gray cast iron, with tensile strengths ranging from 30,000 to 45,000 psi and a Brinell hardness of 187-235. A composition representative of a gray cast iron is C, 3-3.35%; Si, 2-2.4%; S, up to 0.12%; P, up to 0.2%; and Mn, 0.6-0.7%. Chemical and Process Technology Encyclopedia, Iron and Steel (McGraw-Hill 1974).
The majority of cast irons have sufficient silicon (more than 1.5%) so that graphite forms during the solidification process. It is known that the presence of silicon in the cast iron facilitates the dissociation of iron carbide to iron and graphite at high temperatures. The graphite formed gives the metal a gray color on a fractured surface, hence the term gray cast iron. Elements of Materials Science, Graphitization Process (Addison-Wesley 1979). Moreover, the graphite is usually present as flakes within the metal. These graphite flakes reduce not only the tensile properties, but also the ductility of the cast iron. As graphite has almost no strength, the flakes act as voids within the structure and reduce the effective cross-sectional area of the casting. Also, cracks are readily propagated from flake to flake within the metal. Consequently, gray cast iron breaks without perceptible distortion due to its low ductility. Further, gray cast iron has little or no impact or shock-resistance properties. See Marks' Standard Handbook for Mechanical Engineers, Cast Iron, 8th Edition (McGraw-Hill 1978).
In contrast to gray cast iron, nodular cast iron, or ductile cast iron as it is also known, has a metallic structure in which the graphite occurs not in flakes but as tiny balls or spherulites. The presence of a few hundredths of a percent of Mg, Ce or rare earths introduced into the molten iron just before casting causes the spherulite graphite particles to form during solidification.
The formation of spherulite graphite particles is important as the spherical graphite particles do not produce the severe stress concentrations developed internally in gray cast iron. Therefore, much greater ductility is obtained with a nodular cast iron. Due to the lower melting temperature of nodular cast iron as opposed to steel containing no graphite, the use of nodular iron is desirable whenever a low melting temperature and moderate ductility are required. The tensile strength of nodular cast iron is usually in the range of 55,000 to 120,000 psi, and the Brinell hardness in the range of 121 to 273. Chemical and Process Technology Encyclopedia, id.
Compacted graphite cast iron is an iron-carbon-silicon alloy containing graphite in the form of interconnected clusters. The properties of this material are intermediate between those of gray and ductile or nodular cast iron. The correlation between some of the mechanical properties of typical compacted graphite cast irons are illustrated in FIG. 1.
In order to better define the quality of the iron, two quality indices may be used as follows:
1. The product of tensile strength and elongation (TS.times.El): A higher value of this product characterizes a superior iron. PA0 2. The ratio between tensile strength and Brinell hardness (TS/HB): A higher value of this quotient characterizes a superior iron.
Using data provided by K. P. Cooper and C. R. Loper, "Some Properties of Compacted Graphite Cast Iron, AFS Transactions", Vol. 86, 1978, some typical values for the aforementioned quality indices, i.e. (TS.times.El and (TS/HB), were calculated for compacted graphite cast irons having various amounts of silicon. The data obtained from these calculations is shown in Table 1.
TABLE 1 ______________________________________ Typical Mechanical Properties for Iron-Carbon-Silicon Compacted Graphite Cast Irons Tensile Elon- Silicon Strength gation Brinell Content % 1000 psi % Hardness (TS/HB) (TS .times. El) ______________________________________ 1.2 42 3.4 147 0.29 143 2.6 47 6.3 152 0.31 296 ______________________________________
Using the two quality indices described, inspection of Table 1 illustrates that the compacted graphite cast iron containing 2.6% Si is superior to that containing only 1.2% Si.
Due to the fact that the properties of compacted graphite cast iron are intermediate between those of gray cast iron and nodular or ductile cast iron, compacted graphite cast iron is used for many applications. For example, it has been used in the manufacture of ingot molds, flywheels, diesel engine castings, exhaust manifolds, gear pump housings and railroad disc brakes. See D. M. Stefanescu and C. R. Loper, "Recent Progress in the Compacted/Vermicular Graphite Cast Iron Field", Giesserei Praxis, No. 5, 1981. Also see "CG Iron Makes Dramatic Improvement in the Performance of Railroad Disc Brakes", Foote CG I Report No. 4.
However, castings having both thin and thick sections of compacted graphite cast iron have not been produced. If the chemical analysis and molten treatment are adjusted for the average section there is a risk of chilling in the thin sections (smaller than 1/2 inch), due to the high chilling tendency of this particular iron. If increased amounts of ferrosilicon are added in order to counterbalance this chilling tendency, in most cases, high nodularity iron will result, with subsequent losses in the basic properties of compacted graphite cast irons.
Additionally, iron-carbon-silicon compacted graphite cast irons have the disadvantage that in order to produce a pearlitic structure, heat treatment or expensive alloying with copper or tin is required.
Therefore, a need continues to exist for a method for producing compacted graphite cast iron having both thin and thick sections with a reduced amount of nodules contained therein.